1. Field of the Invention
This invention relates to a method of generation and a method of detection of a soliton in an interband phase difference soliton circuit that is one of superconducting circuits and an interband phase difference soliton circuit befitting realization of these methods.
2. Description of the Prior Art
The superconducting electronics that utilize the phase difference among a plurality of superconducting components by means of a multiband superconductor have been disclosed in the first and second prior art references (JP-A 2003-209301 and JP-A 2005-085971) in which the inventors of the present application have taken part.
The bits serving as a base element in their operations are formed by utilizing the quanta behaving as solitary waves, namely interband phase difference solitons (hereinafter abbreviated simply as “soliton” unless otherwise specified). The development of efficient methods of generation and detection of the soliton is a technique that bases these electronics. The term “interband phase difference soliton” as used herein refers to the soliton that is devoid of a quantized magnetic flux (fraxon) producing a motion in a one-dimensionally long Josephson junction. The fraxon accompanies a cyclic electric current within the real space. In contrast, the interband phase difference does not form a quantized magnetic flux because it possesses no cyclic electric current in the real space despite its possession of a cyclic electric current in the wavenumber space (otherwise called “momentum space”).
As regards the generation, however, as disclosed in the first and second prior art references cited above and in the third prior art reference (“Soliton in Two-Band Superconductor,” Y. Tanaka, Physical Review Letters, Vol. 88, Number 1, 017002), a method for creating a boundary condition for the generation of a soliton with a magnetic field has been proposed and, as disclosed in the fourth prior art reference (“Interband Phase Modes and Nonequilibrium Soliton Structures in Two-Gap Superconductors,” A. Gurevich and V. M. Vinokur, Physical Review Latters, Vol. 90, Number 4, 047004), a method for creating a soliton together with an electric current by causing inflow of a nonequilibrium electric current into a superconductor has been proposed.
Meanwhile, concerning the detection, the first to third prior art references disclose a method for detecting the generation of a halfway magnetic flux quantum (Fractional Flux) generated by the soliton and the fourth prior art reference discloses the generation of a voltage due to the counter extinction of a soliton and an antisoliton and a method for detecting a soliton with the voltage that is generated when a soliton is created at an electric current introducing terminal.
The method for generating a soliton by means of boundary conditions dependent on a magnetic field, however, has been at a disadvantage in readily succumbing to the influence of the external field of environment because it is required to use direct mutual action of a soliton and a magnetic field. Further, in the first place, since the “interband phase difference soliton” does not possess the cyclic electric current accompanying a real electric current in the real space, the soliton involved in any of the scenes of the first to third prior art references by nature avoids inducing a mutual action with a magnetic field. Since this fact has been admitted as an advantageous point in the application to a quantum computer, the procedure that necessitates setting boundary conditions dependent on a magnetic field forms a cause for impairing this advantage.
Further, the soliton is created in a closed circuit for the sake of preparing boundary conditions. At this time, the spontaneous electric current that flows in the closed circuit overlaps the soliton. For the purpose of purely extracting the soliton alone, the closed circuit must be built by constructing a switch part within part of the line of the closed circuit and turning off the switch. The formation of this switch part is not simple.
Meanwhile, even when the soliton has been obtained by injecting a nonequilibrium electric current from outside, a method for separating the soliton and the electric current becomes necessary. This method has never been known heretofore.
The detection of the soliton also entails various problems. When a method for detecting a halfway magnetic flux quantum is adopted, a highly advanced technique for determining a magnetic field is required. As means that promises usefulness for the determination of a magnetic flux smaller than the unit quantum magnetic flux, the SQUID microscope disclosed in the fifth prior art reference (“SQUID Microscope,” Toshimitsu Morooka and Kazuo Chinone, Applied Physics, Vol. 70, No. 1, (2001), pp. 50-52) is available. This microscope is at a disadvantage in consuming undue time in the determination of a magnetic flux and suffering the soliton to become extinct before completion of the determination. When the soliton stands still in a faultless or ideal sample, it is topologically stable. The soliton retains life, however, when the sample possesses imperfectness. The life of the soliton shortens particularly when the kinetic energy of the soliton is large. Even for the sake of enabling high-speed electronics, therefore, a means to detect soliton without relying on the method for determining a halfway magnetic flux quantum is needed.
The method that detects the generation of a voltage by the counter extinction of a soliton and an antisoliton as disclosed in the fourth prior art reference may well be called one of powerful methods for the detection of a soliton. This is nevertheless an instance observed during the generation of a soliton induced by the injection of a nonequilibrium electric current from outside. In consideration of the fact that the electric current and the soliton are in a mixed state within the superconductor line allowing running of the soliton, however, the voltage originating in the soliton and the voltage originating in the nonequilibrium electric current injected from outside are not easily separated because they take place in an overlapping state. The voltage signal due to the generation of the soliton at the electrode through which the nonequilibrium electric current is injected is likewise difficult to discern.
For the purpose of applying to the detection of a single soliton the concept of the generation of a voltage due to the counter extinction of a soliton and an antisoliton, it becomes necessary to find a point at which the antisoliton is allowed to flow in. This method has not been known heretofore.
In short, so long as a method capable of rationally and infallibly separating the soliton and the electric current is not available, the voltage signal attendant on the extinction of the soliton is difficult to discern because of the inevitable exposure to the large influence of the electric current. The difficulty encountered by the technique directed to generating and detecting the soliton also renders very difficult the search itself for a superconducting material that enables realizing electronics using the interband phase difference soliton. That is, even when this superconducting material needs to be tested for the purpose of determining whether it can be used for a soliton circuit, a reliable and easy method for performing this test has not been known heretofore.
This invention has been accomplished in view of such true state of prior art as described above and is aimed at providing a method and a circuit that enable accomplishing the object of generating and detecting a soliton without relying on a magnetic flux and with a simple structure, infallibly separating an electric current and a soliton within a superconducting line and dispensing with the susceptibility to the influence of an electric current even during the detection of a soliton.